1. Field of the Invention
This invention relates generally to the field of wireless communication and specifically to ad hoc wireless networks. More particularly, the present invention relates to methods and system for admission control and scheduling in ad hoc wireless networks.
2. Description of the Related Art
A schematic diagram of a typical wireless network is shown in FIG. 1. Here, STA represents a mobile station, also known as a mobile terminal. The physical layer (PHY) and medium access control (MAC) standard for this type of networks are described in “Wireless Local Area Networks, Medium Access Control (MAC) and Physical Layer (PHY) Specifications,” ANSI/IEEE Std 802.11, 1999 Edition. The basic access mechanism specifies two protocols of medium access, the point co-ordination function (PCF) and the distributed co-ordination function (DCF). The DCF medium access is the mode used for the ad hoc mode of operation and carrier sensing multiple access with collision avoidance (CSMA/CA) is the multiple access mechanism. A four way handshake of request to send (RTS), clear to send (CTS), DATA and acknowledgement (ACK) is used for collision avoidance.
The DCF MAC protocol does not provide any priority to any form of traffic, and hence, does not support quality of service (QoS). The IEEE 802.11e standard for QoS enhancements (see “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, MAC Quality of Service (QoS) Enhancements,” IEEE P 802.11e/D 8.0, February 2004) specifies an enhanced DCF (EDCF) mode of operation defining 4 types of access categories (AC). The DCF inter-frame spacing (DIFS) time for clear channel assessment was modified to having a different arbitration inter-frame spacing (AIFS) for clear channel assessment corresponding to each access category. The 802.11e draft also specifies different values of the contention window size corresponding to different access categories, thereby providing the traffic with higher priority, a higher probability of obtaining a transmission opportunity (TXOP). The EDCF mode of operation also does not provide any QoS guarantees to the different classes of traffic. Different applications specify different QoS requirements which need to be provided to the users.
A survey of the existing literature on performance evaluation and admission control in wireless ad hoc networks is as follows (see the list of references appended hereto): In [3], Bianchi developed a discrete time Markov chain (DTMC) based approach to model the back-off stage and the back-off counter value. The DTMC model provided the throughput under saturated conditions. In [4], Carvalho and Aceves used the model developed by Bianchi in [3] to model the delay in single hop 802.11 networks. An expression for the mean and the variance for the “idle time” of a node was derived. The “idle time” was defined as the time a node spends in back-off. The authors in [4] also considered a saturated traffic model or an “always on” model (as in [3]).
Admission control in the DCF and EDCF modes of wireless local area network (WLAN) have been studied in [5]-[7]. In [5], Kanodia et al presented a dynamic priority assignment algorithm in which each node exchanges the priority index of the packet currently under transmission and also the priority index at the head of line or the packet next to the one currently under transmission. The other nodes use this information to update their priority indices. The priority indices are then dynamically varied depending on the time spent by a packet in the system and the number of hops traversed. The mechanism provided in [5] is a modified back-off procedure based on the node priorities. In [6], Valee and Li applied the notion of service curves and used probing packets to predict the service curves. The service curves were obtained by the probing packets by using a moving average of the delay undergone by k waiting packets. However, the approach in [6] was a reactive approach because there was no effective prediction of the channel behavior. In [7], Kuo et al considered the EDCF mode of operation and presented an admission control policy to meet mean delay requirements. An estimate of the mean delay was made using a G/G/1 queuing model. It was found that the admission control procedure was highly conservative and resulted in under utilization of system resources. Also the procedure does not help in meeting delay bound requirements of real time traffic. In [8], Pong and Moors present an admission control approach for differentiated services in WLAN with the IEEE 802.11e EDCF. The contention window sizes were modified based on the throughput requirements of the flows and for each flow, the probability of the collision (and thus throughput degradation) was computed using an exponential weighted average technique.
QoS provisioning in WLAN has been studied widely, some of the studies were mentioned above. While most studies in the current literature consider the infrastructure mode of operation, there are not many efficient mechanisms to provide QoS in the ad hoc mode. The existing literature on admission control in ad hoc networks provides means for exchanging information on the priorities associated with the packets currently under transmission and the packets in the head of line of the nodes. The priority indices of the packets are decided based on the access categories. There are very few studies on providing statistical QoS to packets belonging to different applications. Further, most studies consider the saturated traffic conditions and there are no efficient mechanisms that take into consideration the realistic on-off traffic models and that exploit the properties of session arrival processes.